The pre-painting of metal semimanufactured products is a well-known technology. Metals, for instance, steel and aluminium, are manufactured in continuous foils coiled in rollers, said coils, being pre-painted before the successive cut and forming to obtain manufactured articles for the automotive industry, architectural, household appliances, etc.
The metal sheets before painting are generally subjected to washing operations, the so called pre-treatment phase, to clean the surface. The varnish is layed onto the coil by rollers and usually comprises two or three passages, each of them followed by passing in an oven. The formulations utilized for coating are based on polymers, both curable and non crosslinked.
The industrial coil coating lines are highly productive. They work at high speed, with residence time in the oven lower than one minute (from 15 to 60 seconds). Also in the case of curable coatings, the crosslinking must take place within the above indicated time frame. Therefore, the critical parameter for the film crosslinking is the baking temperature, and more precisely the maximum T reached by the metallic coil, called PMT (peak metal temperature) in the oven. The baking is generally carried out at high temperature, usually chosen in the range of 200-300.degree. C. so as to complete the crosslinking within the indicated short times. At the exit from the oven, the metal sheet is wound in coil. Since the bending stress of the winding exerts very high pressure among the sheets, the polymeric film must be completely cured, i.e. it must have already reached the top of intrinsic mechanical and surface properties. This is needed to avoid apparent surface damages, known, in the technology, as "metal marking" which might compromise the successive processing steps and deteriorate the aesthetic appearance and the duration of the protective effect.
The coatings usually utilized for coil coating of the prior art are curable polyesters, polyesters modified with silicone resins, acrylic modified silicone resins. In the field of coatings which reach final dimensional stability (by crosslinking or melt solidification) in very short times (coil coating conditions), the use of fluorinated polymers is known too, which, thanks to their outstanding chemical and photooxidation resistance, offer high performances both from the aesthetic stand point and the protective function for an extended (often more than 20 years) period of time.
In U.S. Pat. No. 4,314,004 (PPG) the use of formulations for coil coating based on mixtures of PVDF (polyvinylidenefluoride) and acrylic resinsis is described. The painting cycle there described is: metal pretreatment/primer/pigmented coat--by utilizing a PVDF composition (45-85% by weight) and acrylic resin (15-55% by w)--/clear top coat with non pigmented resin based on PVDF (45-85% by w) and acrylic resin (15-55% by w).
The acrylic resin secures the intercoat adhesion. The obtained films have high chemical and weather resistance (4300 h QUV test, delta E=1.0 and delta gloss retention &gt;80%).
Due to the PVDF intrinsic characteristics (semicrystalline) the gloss of the PVDF-acrylic coating, measured at 60.degree., does not exceed values of about 40.
To obtain higher gloss, the use of other fluorinated polymers is indicated in the prior art. For instance U.S. Pat. No. 5,178,915 (Morton) describes the use of transparent formulations, for the application on anodized metal and for coil coatings, containing resins such as LUMIFLON.RTM., CEFRAL.RTM., i.e. polymers obtained by polymerization of chlorotrifluoroethylene (CTFE) with hydrogenated vinylethers or vinylesters, crosslinked for instance with blocked isocyanates and/or melamines and formulated with various kinds of additives (UV-adsorber, antifoaming, stretching agents, antiscratch, etc.) to improve the final properties of the film. The crosslinked fluorinated resins offer, besides higher values of gloss (at 60.degree., values of about 80) also good bending resistance (T bend=2T), remarkable chemical and ageing-resistance, i.e. performances not far from those of PVDF. The performace similarity and complementarity have been shown in U.S. Pat. No. 5,366,803 (Mitsubishi). Indeed transparent and pigmented formulations containing LUMIFLON.RTM. resins, crosslinked with melamines and applied onto PVDF coated coils are described therein. The crosslinked films have high gloss (in the range of 80) and UV resistance (2000 hours QUV test).
The weak points of the coatings utilized in the prior art are represented by the following facts:
poor resistance to the metal marking with consequent deterioration of the optical characteristics of the coating, such as gloss and definition of image (DOI) PA1 poor bending resistance, both of the transparent coating and, especially, of the pigmented one, which is apparent in the hazing (yield) and in the brittle splitting with consequent loss of film continuity and, therefore, of the protective anticorrosion characteristics PA1 very poor characteristics of stain release and of no dirt pick-up.
The coatings of the prior art, on the average, show lack of all these three features. More precisely they can, at most, improve a single performance. However, this is achievable with substantial draw backs affecting the other required fundamental features.
Moreover, and in particular for the last feature listed above (stain release and no dirt pick-up), the art teaches that this lack affects every kind of coating for whatsoever application technology. Antistatic additives are commonly used and are capable of partially moderate this weakness. The improvement, however, is lost after a short time of exposure to the environment because of the combined attack of pollutants, environmental aggressive agents and UV exposure.
The fluorinated polymers themselves, notwithstanding the superior characteristics of chemical resistance and the theoretically excellent surface properties of the fluorine, do not offer acceptable performances to satisfy this requirement. Indeed, the fluorinated polymers tend to electrostatically charge themselves and therefore to attract the environmental particulate; moreover the well known fluorocarbon water- and oil-repellence does not automatically mean an equivalent capacity of the stains repellence.
The stains and the dirt are a complex combination of substances of various kind, both of natural and of human origin. Among the components we have metals, metal oxides and hydrates, salts, carbon and carbonaceous and organic degraded substances (partially oxidized). The chemical-physical polyvalence of this variable combination of substances makes very difficult whatever attempt of comprehensive schematization and quantified evaluation of the phenomena. There is no consensus opinion on test conditions representative of the real world. This does not modify the apparent fact that the polymeric coatings easily soil. The matter is under the eyes and the attention of everybody, since also an aesthetic function (stain resistance) as well as a protective function of the substrates are required thereto.
To improve the stain release of fluorinated and non fluorinated coatings, in the prior art the use of polyfunctional molecules is indicated. For instance in EP patent 700,957 complex molecules incorporating different combined structures are cited, in particular the combination of silicone polymeric chains with fluorinated polymeric chains, bound to hydrogenated blocks containing various chemical functions (hydroxylic, aminic, carboxylic, polyethoxylic, etc.) These systems show only a partial improvement of the stain release characteristics and above all are not capable of maintaining the performance during the time.